Here are some things to look for in a good system for this hobby:
No more than 4 stages (sediment, carbon, RO, DI) unless you have specific water quality issues you want to address
Standard-sized prefilters, membrane, and DI cartridge
Brand name, high-rejection membrane
Specifications provided for each stage
Pressure gauge after the prefilters and before the membrane
Thermometer
TDS meter (handheld or in-line)
DI bypass
Vertical DI stage
Refillable DI cartridge
Aluminum bracket
Quick connect fittings
No water from pressurized storage container delivered to DI
Flush valve
Clear housings
High-quality instructions written in clear English
Customer support before, during, and after your purchase
A vendor involved in your hobby
Russ